In many industrial environments the use of pressurised piping is commonplace, such as oil rigs, refineries, gas production and storage facilities and power generation plants which usually contain complex pipe networks to move a variety of hazardous and non-hazardous high pressure media such as gas and liquids. The pipes can also be used for operating or controlling processes as well as distribution of process and end gas product. The gas can potentially be toxic or explosive which may require immediate action although any pressurised gas leak should be dealt with in the utmost of urgency and therefore requires careful monitoring of leaks.
Other industrial operations which produce pressurised gas or which rely on pressurised gas as part of machine operation contain mechanical parts which may produce ultrasound when components wear or are under stress.
Historical ultrasonic noise evaluation of most industrial sites indicates that most man-made noise occurs in the acoustic range whereas a pressurised gas leak produces a broadband sound pressure level that spans the acoustic and ultrasonic range. The size of the ultrasonic sound signal is a function of many variables which include the upstream to downstream pressure differential, leak size, gas type and gas temperature and type of pressurised system, driven or closed. It is important to establish whether a system is driven using a pressure generator, such as a pump, to maintain system pressure, or if the system is closed, such as a pressure vessel, to establish the potential leak profile intensity and duration. If the system is driven by a constant pressure the leak profile will remain reasonably constant for the leak duration with minor fluctuations across the frequency range whereas a closed system will experience rapid cooling due to energy transfer and the sound pressure level will diminish rapidly across the frequency range with minor fluctuations which requires instantaneous detection to avoid gas pools going undetected. It should be noted that although no two gas leaks will produce an identical profile the broadband characteristic which decreases in intensity with increasing frequency allows the leaks to be susceptible to detection.